D19.2 Standard Formation Enthalpy
Standard formation enthalpy, ΔfH°, is the enthalpy change for a reaction in which exactly one mole of a pure substance in a specified state (s, l, or g) is formed from free elements in their most stable states under standard-state conditions. ΔfH° is also referred to as the standard heat of formation.
For example, ΔfH° of CO2(g) at 25 °C is −393.5 kJ/mol. This is the enthalpy change for the exothermic reaction:
The gaseous reactant and product are at a pressure of 1 bar, the carbon is present as solid graphite, which is the most stable form of carbon under standard-state conditions.
For nitrogen dioxide, NO2(g), ΔfH° is 33.2 kJ/mol at 25 °C:
A reaction equation with ½ mole of N2 and 1 mole of O2 is appropriate in this case because the standard enthalpy of formation always refers to formation of 1 mole of the substance, here, it is NO2(g).
By definition, the ΔfH° of an element in its most stable form under standard conditions is 0 kJ/mol. A table of ΔfH° values for many common substances can be found in the Appendix.
Activity: Equations for Standard Formation Enthalpy
Hess’s law can be used to determine the ΔrH° of any reaction if the ΔfH° of the reactants and products are available. In other words, we can think of any reaction as occurring via step-wise decomposition of the reactants into their component elements followed by re-combination of the elements to give the products. (Almost no reaction would actually occur via such a mechanism, but we can imagine such a path for the sole purpose of using Hess’s law to calculate ΔrH°.)
The ΔrH° of the overall reaction is therefore equal to:
Exercise: Using Standard Formation Enthalpies
Activity: Applying Hess’s Law to Standard Formation Enthalpies
Please use this form to report any inconsistencies, errors, or other things you would like to change about this page. We appreciate your comments. 🙂
